Method for measuring the modulation error of digitally modulated high frequency signals

ABSTRACT

A method is described for measuring the modulation error in digital modulated high frequency signal, whose amplitude and/or phase is digitally distorted according to a predetermined function, in particular limited by a given clip factor, in which the signal to be measured is compared with a locally generated reference signal that was generated by taking the predetermined distortion function into account.

[0001] The invention relates to a method for measuring the modulationerror of a digitally modulated high frequency signal, as used, forexample, in modern mobile telephone and radio systems.

[0002] For measurements of this kind, measuring devices are alreadyknown, wherein an ideal reference signal is generated locally from aknowledge of the bits and the modulation parameters used for the digitalmodulation of the signal to be measured, such as type of modulation,filter function and symbol rate. The signal to be measured is thencompared with the ideal reference signal (e.g. U.S. Pat. No. 5,001,724).In this context, the ideal modulated reference signal is generated inthe measuring device either by synthesis or by calculation. With amodulation analyser of this kind, the modulation error of a highfrequency signal digitally modulated in any manner required can bemeasured and presented separately according to amplitude and phase as anI-error, a Q-error, vector-error amplitude or in a polar presentation asa vector diagram or configuration diagram.

[0003] One disadvantage of these known measuring devices is that whenmeasuring signals, of which the amplitude or phase is distortedaccording to a known function on the digital side in the base band, forexample, by being limited to a predetermined clip factor of 64%, arelatively high modulation error is already measured with reference tothese signal distortions alone. If, for example, the modulation error ofa transmitter power amplifier is to be measured with a known measuringdevice of this kind, and a distorted high frequency signal of this kind,for example, a clipped signal, is used for this purpose, the modulationerror caused by the power amplifier, which is actually supposed to bemeasured, remains covered beneath the modulation error caused by thedistorted measuring signal and can no longer be measured accurately.High frequency signals, which are distorted, for example, clipped,according to a known function, are increasingly used in modern radiosystems for various reasons, and the modulation error can accordingly nolonger be measured in the above manner. This applies in particular toclipped high frequency signals such as those used in the mobiletelephone system IMT2000 of the next mobile telephone generation and/orin digital radio and television, where the modulation error of systemcomponents is supposed to be measured directly using the known measuringdevices named above.

[0004] The object of the present invention is therefore to provide amethod, wherein the modulation error of digitally modulated signals, ofwhich the amplitude and/or phase is distorted on the digital sideaccording to a known function, especially by being limited to apredetermined clip factor, can also be measured.

[0005] This object is achieved on the basis of a method according to thepreamble of the independent claim by its characterising features. Thedependent claims define advantageous further embodiments.

[0006] When generating the ideal reference signal according to theinvention by calculation or synthesis from the known sequence of thesignal to be measured, not only are the modulation parameters taken intoconsideration in the known manner; the distortion function used in thesignal to be measured is additionally taken into account. Accordingly,the measured modulation error attributable to distortion remainsminimal; the modulation error caused, for example, by a power amplifierof a WCDMA base station, can be accurately measured in a clipped signal,because it is no longer covered by the modulation error caused as aresult of clipping. The method according to the invention can be usedboth with single-carrier and also with multiple-carrier high frequencysignals, which are digitally modulated. It can be used for all signals,which are limited on the digital side to a predetermined clippingfactor. In the case of WCDMA signals according to the 3GPP standard, thebase band signal is limited before filtering, for example, by clippingto 64% of the full amplitude in order to keep the crest factor of thesignal as low as possible, which benefits the efficiency of the poweramplifier used in the transmitter. Since different clip factors areconventional for each of the various types of signal, for which themethod according to the invention is suitable, this parameter ispreferably entered manually into the measuring device and is then takeninto account during the calculation of the reference signal. The methodaccording to the invention can therefore be used by a simple extension,for example, in the signal analyser FSIQ manufactured by Rohde &Schwarz. For the internal calculation of the ideal reference signal inthe measuring device, it is only necessary for the user to enter thisclip factor, for example, in the case of WCDMA signals, of 64% of thefull amplitude. This factor will then be taken into consideration in thecalculation of the reference signal, and the vector or the I-valuesand/or Q-values of the reference signal can be calculated back.

[0007] The high frequency signal could also, for example, be limited bya so-called soft-clipping function, which is entered into the measuringdevice. In the same manner, when the reference signal is generated inthe measuring device, other known signal distortion functions can betaken into consideration, for instance, functions for compressing highfrequency signals, such as those used for various reasons in modernradio systems in the context of digital signal conditioning.

1. A method for measuring a modulation error in a digitally modulatedhigh frequency signal of which the amplitude and/or phase has beendistorted on a digital side according to a predetermined function,comprising: locally generating an ideal reference signal; distorting thelocally generated, ideal reference signal in accordance with saidpredetermined function; and comparing the signal with the distorted,locally generated, ideal reference signal.
 2. A method according toclaim 1, wherein: the ideal reference signal is generated locally in themeasuring device by calculation, and the predetermined function can beset manually on a measuring device.
 3. A method according to claim 1,wherein: the amplitude of the signal is limited on the digital side by apredetermined clip factor, and said predetermined function includesclipping the locally generated ideal reference signal in accordance withthe predetermined clip factor.
 4. A method according to claim 2,wherein: the amplitude of the signal is limited on the digital side by apredetermined clip factor, and said predetermined function includesclipping the locally generated ideal reference signal in accordance withthe predetermined clip factor.
 5. A method according to claim 1,wherein: said predetermined function includes a soft clipping functionentered into a measuring device.
 6. A method according to claim 1,wherein: said predetermined function includes high frequency signalcompression.
 7. A measurement device for measuring a modulation error ina digitally modulated high frequency signal, of which the amplitudeand/or phase has been distorted on a digital side according to apredetermined function, comprising: a signal generator for generating anideal reference signal; means for distorting the ideal reference signalin accordance with said predetermined function; and means for comparingthe signal with the distorted, locally generated, ideal referencesignal.
 8. A measurement device according to claim 7, wherein: the idealreference signal is generated locally in the measuring device bycalculation, and the predetermined function is set manually on themeasurement device.
 9. A measurement device according to claim 7,wherein: the amplitude of the signal is limited on the digital side by apredetermined clip factor, and said predetermined function includesclipping the locally generated ideal reference signal in accordance withthe predetermined clip factor.
 10. A measurement device according toclaim 8, wherein: the amplitude of the signal is limited on the digitalside by a predetermined clip factor, and said predetermined functionincludes clipping the locally generated ideal reference signal inaccordance with the predetermined clip factor.
 11. A measurement deviceaccording to claim 7, wherein: said predetermined function includes asoft clipping function entered into the measurement device.
 12. Ameasurement device according to claim 7, wherein: said predeterminedfunction includes high frequency signal compression.